avoid quadratic quote stripping in MultiPartName.unquote

Author: tanjiroK-coder

src/main/java/net/sf/jsqlparser/schema/MultiPartName.java

@@ -23,13 +23,34 @@ public interface MultiPartName {
      * @return the pure identifier without quotes
      */
     static String unquote(String quotedIdentifier) {
-        return quotedIdentifier != null
-                ? LEADING_TRAILING_QUOTES_PATTERN.matcher(quotedIdentifier).replaceAll("")
-                : null;
+        if (quotedIdentifier == null) {
+            return null;
+        }
+        // same result as LEADING_TRAILING_QUOTES_PATTERN.matcher(..).replaceAll(""), but linear:
+        // the regex retries its trailing alternative at every offset of a run of quotes
+        int start = 0;
+        int end = quotedIdentifier.length();
+        while (start < end && isLeadingQuote(quotedIdentifier.charAt(start))) {
+            start++;
+        }
+        while (end > start && isTrailingQuote(quotedIdentifier.charAt(end - 1))) {
+            end--;
+        }
+        return quotedIdentifier.substring(start, end);
     }
 
     static boolean isQuoted(String identifier) {
-        return identifier != null && LEADING_TRAILING_QUOTES_PATTERN.matcher(identifier).find();
+        return identifier != null && !identifier.isEmpty()
+                && (isLeadingQuote(identifier.charAt(0))
+                        || isTrailingQuote(identifier.charAt(identifier.length() - 1)));
+    }
+
+    private static boolean isLeadingQuote(char c) {
+        return c == '"' || c == '[' || c == '`';
+    }
+
+    private static boolean isTrailingQuote(char c) {
+        return c == '"' || c == ']' || c == '`';
     }
 
     String getFullyQualifiedName();

src/test/java/net/sf/jsqlparser/schema/MultiPartNameTest.java

@@ -9,6 +9,9 @@
  */
 package net.sf.jsqlparser.schema;
 
+import java.time.Duration;
+import net.sf.jsqlparser.parser.CCJSqlParserUtil;
+import net.sf.jsqlparser.statement.select.PlainSelect;
 import org.assertj.core.api.Assertions;
 import org.junit.jupiter.api.Test;
 
@@ -21,4 +24,31 @@ void replaceBackticksWithDoubleQuotes() {
         Assertions.assertThat("\"starbake\".\"customers\"").isEqualToIgnoringCase(
                 MultiPartName.replaceBackticksWithDoubleQuotes("`starbake`.`customers`"));
     }
+
+    @Test
+    void unquoteMatchesQuotePattern() {
+        String[] names = {"", "a", "\"a\"", "[a]", "`a`", "\"a\".\"b\"", "\"\"\"\"", "\"]",
+                "[\"a\"]", "a\"", "\"a", "]a[", "``a``", "\"a\"\"b\"", "[[a]]"};
+        for (String name : names) {
+            assertEquals(MultiPartName.LEADING_TRAILING_QUOTES_PATTERN.matcher(name).replaceAll(""),
+                    MultiPartName.unquote(name), name);
+            assertEquals(MultiPartName.LEADING_TRAILING_QUOTES_PATTERN.matcher(name).find(),
+                    MultiPartName.isQuoted(name), name);
+        }
+        assertNull(MultiPartName.unquote(null));
+        assertFalse(MultiPartName.isQuoted(null));
+    }
+
+    @Test
+    void unquoteIsLinearOnRunsOfEscapedQuotes() {
+        // a quoted identifier with a dot and many doubled quotes used to backtrack quadratically
+        String quotes = "\"\"".repeat(40000);
+        String sql = "SELECT * FROM \"a" + quotes + ".b\"";
+        PlainSelect select = (PlainSelect) assertTimeoutPreemptively(Duration.ofSeconds(2),
+                () -> CCJSqlParserUtil.parse(sql));
+        assertNotNull(select.getFromItem());
+        assertEquals("a" + quotes + ".b",
+                assertTimeoutPreemptively(Duration.ofSeconds(2),
+                        () -> MultiPartName.unquote("\"a" + quotes + ".b\"")));
+    }
 }